There is a need in both the telecommunications industry and the computer systems industry for output drivers to drive transmission lines. When driving transmission lines it is desirable for the output impedance of the driver to match the characteristic impedance of the transmission line as closely as possible in order to minimize reflections that cause ringing, overshoot, and undershoot at a receiver. Parallel terminated drivers such as ECL type or pseudo-ECL type drivers provide a controlled output impedance; however, they suffer from excessive power dissipation due to the substantial standby current in the parallel termination resistor. For this reason, series terminated drivers are favorable because the excessive power dissipation is eliminated. The series terminated driver is uniquely suited for low power, high frequency, point-to-point transmission.
In providing terminated drivers, the appropriate output impedance may be configured either "on-chip" or off-chip. Off-chip resistance solutions allow one to precisely configure the impedance, but forces one to allocate board space and requires discrete components which add costs. On-chip resistance solutions circumvent the off-chip solution problems. However, prior art on-chip solutions require a trimming process to ensure a precise impedance value because semiconductor processes cannot ensure high precision resistors. The trimming process is an extra step in manufacture and adds to the cost of manufacture.
It is an object of this invention to provide a method and circuit that provides a low power, point-to-point series terminated output driver with a controlled output impedance that can operate at high frequencies with no off-chip resistances and with no trimming process. Other objects and advantages of the invention will become apparent to those of ordinary skill in the art having reference to the following specification together with the drawings herein.